Problem: In chemistry class, the girl to boy ratio is $2$ to $6$. If there are a total of $40$ students, how many girls are there?
Answer: A ratio of $2$ girls to $6$ boys means that a set of $8$ students will have $2$ girls and $6$ boys. A class of $40$ students has $5$ sets of $8$ students. Because we know that there are $2$ girls in each set of $8$ students, the class must have $5$ groups of $2$ girls each. There is a total of $10$ girls in chemistry class.